Rubber compounding involves the use of a great variety of components which eventually result in a cured rubber article such as a tire. Typically, rubber compounds for the application are cured by a sulfur vulcanization process. This process has long been known to be influenced by the addition of numerous reagents. One class of reagents includes fatty acids commonly used in conjunction with metal oxides such as zinc oxide to improve sulfur vulcanization efficiency. However, the metal oxides and fatty acids are generally added in specific and determinable amounts.
Metal oxides and other metal salts and complexes have also been added to cause certain enhancements of particular rubber properties. U.S. Pat. No. 4,684,421 teaches the use of calcium borate in conjunction with a cobalt or nickel carboxylate to improve rubber to wire adhesion.
Rubber compounds for use in tires, belts and other types of rubber products, can contain a wide variety of different types of ingredients combined to form a final cured product. The final product is generally a highly crosslinked, highly filled rubber or rubber composite. Commonly, these rubber compounds are formulated by mixing together under masticating conditions ingredients including, but not restricted to, extender oils, bulk elastomers, fillers, antidegradants, curatives, cure activators, waxes and plasticizers. The final products are then generally cured thermally to yield a given product such as a tire, belt, hose or any other product containing cured rubber.
Certain ingredients can significantly affect vulcanization and ultimately final rubber product properties. These ingredients can also affect other properties such as tack (rubber to rubber or rubber to other ingredient adhesion) and product uniformity. Fatty acids are one such ingredient. Fatty acids may be present in the bulk elastomer prior to compounding. Fatty acids are also typically added to elastomers during compounding to augment cure behavior and to increase the state of cure. It is believed that fatty acids increase the solubility of zinc oxide allowing a greater amount of zinc salt to participate in sulfur vulcanization.
Although fatty acids are routinely added to elastomers during compounding to achieve improved vulcanizates, excess fatty acid content can cause a decrease in tack. Tack is related to the ability of uncured rubber compounds to adhere to components including itself (autohesion) and other rubber compounds. Tack is therefore vital to any type of building process involving rubber composites or mixed composites containing rubber, fiber, fabric, metals or other ingredients. It is thought that fatty acids bloom to the surface of a rubber compound after mixing (migrate to the surface of a rubber component) which causes an increase in fatty acid content near or at the surface of the compounded rubber. This build up of fatty acid at or near the surface of a rubber is thought to significantly and measurably reduce tack.
Bulk elastomers, which have either naturally occurring fatty acids or fatty acids added during manufacture, usually have amounts of fatty acids which are difficult to determine. The presence of an indeterminate amount of fatty acids in a bulk elastomer makes strict control of vulcanization difficult. This lack of strict control can affect final product uniformity as well as final product properties. Analysis of fatty acid content by classical methods, e.g., extraction and chromatography of the extract, is too cumbersome for plant use.
In order to make uniform rubber products and have tight controls on vulcanization utilizing elastomers that have variable or indeterminate amounts of fatty acids, a method for deactivating the fatty acids present in bulk elastomers is desirable.